This invention relates to a projection exposure apparatus used for exposing a pattern on a mask onto a photosensitive substrate in a photolithographic process for manufacturing a semiconductor device, liquid crystal display device, image pick-up device (CCD), thin-film magnetic head, and the like, and more particularly to a projection exposure apparatus having a mechanism for correcting the imagery characteristic of the projection lens system.
For manufacturing a semiconductor device or the like, a projection exposure apparatus is generally used, which transfers a pattern image formed on a reticle (serving as a mask) onto a wafer (or glass plate) as a photosensitive substrate through a projection lens system. Conventionally, an exposure apparatus of a collective exposure type, such as a stepper, has been used. However, a so-called step-and-scan type projection exposure apparatus has been recently substituted for the collective exposure type projection exposure apparatus. The step-and-scan type exposure apparatus exposes a pattern image onto a shot area on the wafer, while scanning both the reticle and wafer with respect to the projection lens system.
The projection lens system used in a projection exposure apparatus requires high resolution over substantially the entire exposure area, because the circuit pattern of the reticle must be precisely transferred onto the wafer. To this end, measures have been proposed to correct aberration in the projection lens system in every stage of the design and manufacturing processes. However, because the imagery characteristic of the projection lens system easily varies in response to the changes in atmospheric pressure, environmental temperature, absorption of illumination, etc., merely satisfying a certain imagery characteristic under a specific environmental condition is insufficient in practical use.
Recent projection exposure apparatus are equipped with an imagery characteristic correction mechanism, which measures the fluctuation in parameters of environmental conditions to calculate the changing amount of the imagery characteristic, or alternatively, directly measures the changing amount of the imagery characteristic and corrects the imagery characteristic of the projection lens system. The imagery characteristic of the projection lens system may be intentionally changed so that the apparatus matches with the characteristics of other projection exposure apparatus or photosensitizer.
Examples of the technique for correcting the imagery characteristic using an imagery characteristic correction mechanism include a method for driving the optical elements (lenses) in the projection lens system or the reticle along the optical axis of the projection lens system to correct the projection magnification, isotropic distortion (barrel distortion), spherical distortion, image plane distortion, etc. Another technique is to tilt the lens element of the projection lens system or the reticle with respect to a plane perpendicular to the optical axis of the projection lens system to correct anisotropic distortion (trapezoidal distortion) and image plane inclination. Still another technique is to seal the gap between certain lenses of the projection lens system and change the internal pressure of the sealed space to adjust the refractive index of the internal air to correct the projection magnification, isotropic distortion (barrel distortion), spherical aberration and image plane distortion.
When using such a conventional imagery characteristic correction mechanism, the imagery characteristic of the projection optical system is appropriately corrected; however, unintended positional shifts of the image-forming position may occur. This is because when driving the driven object (particularly lenses within the projection lens system, or the reticle) along the optical axis of the projection lens system, the driven object often slightly slips out of the optical axis and advances obliquely because it is difficult to drive the lens or reticle strictly parallel to the optical axis. Generally, when driving the lens or reticle in the optical-axis direction, the position of the lens or the reticle is strictly controlled by, for example, a position sensor. On the contrary, with respect to the direction perpendicular to the optical axis, the movement of the driven object is simply guided by a guide mechanism because it is typically not necessary to control the movement of the lens in the direction perpendicular to the optical axis. However, slackness (vibration) or elastic deformation of the guide mechanism may cause the driven object to move slightly out of line with the optical axis, which may result in displacement of the image-forming position of the pattern image off the optical axis.
During a semiconductor manufacturing process, multiple layers of different circuit patterns are exposed onto the wafer. Each pattern must be precisely superimposed on the previous pattern formed through the previous exposure. The projection exposure apparatus generally has an alignment sensor for detecting a registration mark formed on the previous pattern to determine a proper exposure position. Examples of the alignment sensor include a TTR (through-the-reticle) sensor, which monitors both the alignment mark on the reticle (referred to as a reticle mark) and the alignment mark on the wafer (referred to as a wafer mark) simultaneously. While a TTR sensor is very precise, there are several limitations in its operation because of the simultaneous measurement of the reticle and the wafer. To this end, an off-axis method is often used, in which only the wafer mark is detected using an alignment sensor fixed to the side of the projection lens system. In the off-axis method, the positional relation (base-line amount) between the reticle mark (more precisely, the center of the projected pattern image of the reticle) and the detection center of the alignment sensor is obtained and stored in advance. When the alignment sensor detects the position of the wafer mark, displacement of the wafer during exposure is then determined based on the detection result of the sensor and the prestored positional relation.
When exposing multi-layers of circuit patterns on the photosensitive substrate, the mark formed on the photosensitive substrate is aligned with the mark-detection optical system. The substrate stage is then moved from this position by the base-line amount to execute exposure. In this manner, the reticle pattern image is aligned with the circuit pattern, which has already been formed on the photosensitive substrate.
In a conventional projection exposure apparatus, the positional relationship between the image position of the reticle mark projected on the substrate stage and the detection center of the mark-detection optical system must be accurately detected. The absolute position of the reticle in a projection exposure apparatus is not so strictly regulated, as long as the reticle position relative to the photosensitive substrate is precisely controlled. For example, the reticle position relative to the projection optical system is not strictly controlled as long as the reticle is positioned within the exposure area and the precision of the projection optical system is assured in that area. A projection optical system is generally composed of a plurality of (e.g., twenty or more) lens elements. The optical axis of the projection optical system is defined by a composition of offset components of the center axes of the respective lens elements. It is not defined by the outer diameter of the projection optical system or the positions of the lens elements. If the area of reticle positions relative to the projection optical system is too large, the exposure area of the projection optical system must also be set large, which increases the cost.
In view of the circumstance described above, the position of the reticle is conventionally adjusted with reference to the outer diameter of the lens barrel of the projection optical system at a mechanical precision of about 200-400 xcexcm.
If the image-forming point changes through driving the imagery characteristic correction mechanism after the relation between the reticle mark and alignment sensor has been stored, then the wafer position slips out of the proper position, which causes an alignment error in superimposing pattern layers.
Occurrence of an alignment error is not limited to the case in which the lens element(s) or reticle are driven along the optical axis. For example, in the method in which the gap between the lens elements in the projection lens system is sealed to change the internal pressure to adjust the refractive index, the retainer supporting the lens elements may elastically deform due to the internal pressure, which causes the lens element to slightly move on or out of the optical axis. If the lens element moves in the direction perpendicular to the optical axis, the image-forming position would slip out of the proper position.
In addition to such unintended change in the image-forming position as descried above, there is also implicit error in the system. For example, when the driven object is tilted with respect to a plane perpendicular to the optical axis of the projection lens system, anisotropic distortion may change, and at the same time, the entire image may shift. If, after distortion is corrected by tilting the driven object, the driven object is further driven along the optical axis to correct the isotropic distortion, then the image-forming position slightly changes even if the driven object is moved precisely along the optical-axis direction, because the driven object is already tilted.
Thus, when the imagery characteristic is corrected by driving the imagery characteristic correction mechanism, the image-forming position changes due to the imperfection of the driving mechanism, resulting in alignment errors.
Moreover, as the circuit patterns become smaller and more detailed, the requirement for alignment precision becomes stricter. To this end, in a recent technique, the magnification of the projection optical system is adjusted to correct the distortion of the photosensitive substrate. The position of the detection mark on the photosensitive substrate is detected to determine an amount of expansion (or contraction) of the photosensitive substrate. The magnification of the projection optical system is adjusted to the optimum value taking into consideration the expansion (or contraction) of the photosensitive substrate, so that exposure is performed under the optimum magnification, correcting the thermal distortion of the photosensitive substrate due to the high-temperature process.
When the magnification of the projection optical system is slightly changed through magnification adjustment, however, the center of the reticle pattern image also slightly shifts because the center of the reticle is not in precise alignment with the optical axis of the projection optical system in the conventional apparatus. When the reticle center is not coincident with the optical axis of the projection optical system, it is also offsets from the alignment sensor within the projection exposure apparatus. Even though the magnification is set to the optimum state through the adjustment operation, the pattern image, which is to be superimposed onto the previous circuit pattern layer on the substrate, shifts as a whole.
As shown in FIG. 8, the center RCT of the reticle pattern image is offset from the optical axis AX of the projection optical system (in the effective area 201). In this example, the projection magnification is adjusted according to expansion of the photosensitive substrate, and the initial reticle image 202 is enlarged to the reticle image 203. The reticle image expands apart from the optical axis AX, which is a fixed reference axis, in proportion to the magnification. The vertices of the reticle image move outward along the dashed lines 204, 205 and 206, and the center RCT of the reticle image also slightly moves along the dashed line 204 because the center RCT of the reticle image is initially offset from the optical axis. This positional shift results in an alignment error.
When the position of the reticle is adjusted mechanically with reference to the outer diameter of the projection optical system, the center of the reticle pattern image generally shifts about 200-400 xcexcm from the optical center axis. If the magnification is changed by 20 ppm in this state, the center of the reticle pattern image shifts 4-8 nm. Alignment accuracy in recent projection exposure apparatus requires that the error be within 100 nm. Considering various other factors of alignment errors, such as imagery characteristic (mainly distortion) of the projection optical system, variation in the base-line amount, fluctuation in the stage control accuracy, or measurement error in the alignment sensor (mark detection optical system), it is not acceptable that the error caused by magnification adjustment occupies almost one tenth of the entire acceptable error range.
As a magnification adjusting mechanism, a mechanism for driving a part of the lens elements of the projection optical system along the optical axis is known. It is very difficult, however, to accurately drive the lens elements along the optical axis. A lens element slightly slips out of alignment with the optical axis in the direction perpendicular to the optical axis, or tilts with respect to the optical axis, which causes the reticle pattern image to shift. In this situation, positional shift of the reticle pattern image due to offset of the lens elements is further added to the divergence due to offset of the reticle pattern image center from the optical center axis, and the alignment error caused by magnification adjustment becomes still worse.
This invention aims to provide a projection exposure apparatus having an imagery characteristic correction mechanism that corrects the imagery characteristic of the projection lens system and is capable of suppressing occurrence of an alignment error in superimposing pattern layers even if the imagery characteristic correction mechanism is driven.
These and other objects of the invention are achieved by providing a projection exposure apparatus for projecting a pattern image formed on a mask onto a photosensitive substrate though a projection optical system to form a projected image thereon. The projection exposure apparatus includes a substrate position detector that detects a position of a registration mark formed on the substrate, an imagery characteristic correction mechanism coupled with the projection optical system that drives the projection optical system to correct an imagery characteristic of the projection optical system, an image-forming displacement detector communicating with the imagery characteristic correction mechanism that determines a displacement amount of an image-forming position of the projected image formed through the projection optical system in accordance with a driven amount of the projection optical system by the imagery characteristic correction mechanism, and an alignment signal processor communicating with the substrate position detector and the image forming displacement detector, wherein the alignment signal processor corrects the detection result of the substrate position detector based on the displacement amount of the image-forming position obtained by the image-forming displacement detector.
The imagery characteristic of the projection lens can be corrected by the imagery characteristic correction mechanism. The image-forming position of a projected image formed through the projection optical system may be displaced by driving the imagery characteristic correction mechanism, which displacement can be detected by the image-forming displacement detector. The displacement amount detected by the image-forming displacement detector is used by the alignment signal processor to correct the detection result of the substrate position detector. With this structure, even if the image-forming position is displaced by driving the imagery characteristic correction mechanism, the displacement does not result in an alignment error in superimposing pattern layers.
The image-forming displacement detector preferably accesses a memory storing the relation between the driving amount of the projection optical system by the imagery characteristic correction mechanism and the displacement of the image-forming position of the projected image formed through the projection lens. When the imagery characteristic correction mechanism is driven, and the image-forming position changes as a result, displacement of the image-forming position is instantaneously detected based on the prestored relation. The displacement can be automatically corrected based on the prestored relation.
The imagery characteristic correction mechanism preferably includes a driving device coupled with one or more optical elements of the projection lens system and/or the mask to drive the optical elements or the mask along the optical axis, or, alternatively, with respect to a plane perpendicular to the optical axis.
The imagery characteristic correction mechanism may alternatively control a gas pressure adjustment mechanism for changing the internal pressure within the sealed space between the lens elements of the projection lens system.
In another aspect of the invention, an exposure apparatus projects a transfer pattern image formed on a mask through a projection lens system onto a photosensitive substrate to form a projected pattern image thereon. The projection exposure apparatus includes a substrate position detector that detects a position of the registration mark on the substrate, an imagery characteristic correction mechanism coupled with the projection optical system that drives the projection optical system to correct an imagery characteristic of the projection optical system, a base-line amount measuring device that measures a distance between a detection center of the substrate position detector and a center of the projected image formed through the projection optical system, the distance defining a base-line amount, and an alignment signal processor communicating with the substrate position detector and the base-line amount measuring device that corrects the detection result of the substrate position detector based on the base-line amount.
The imagery characteristic of the projection lens system is corrected by the imagery characteristic correction mechanism. The position of the mask pattern image may be displaced when the imagery characteristic is corrected, and the displacement is reflected in variations in the base-line amount. The base-line amount can be measured again by the base-line amount measuring mechanism after the base-line amount has changed, and the detection result of the substrate position detector is corrected based on the remeasured base-line amount. Thus, even if the image-forming position is laterally displaced by driving the imagery characteristic correction mechanism, alignment errors can be suppressed in superimposing pattern layers.
In accordance with still another aspect of the invention, there is provided a method of projecting a pattern image formed on a mask onto a photosensitive substrate through a projection optical system to form a projected image thereon. The method includes the steps of (a) detecting with a substrate position detector a position of a registration mark formed on the substrate, (b) driving the projection optical system to correct an imagery characteristic of the projection optical system, (c) determining a displacement amount of an image-forming position of the projected image formed through the projection optical system in accordance with a driven amount of the projection optical system in step (a), and (d) correcting the detected position from step (a) based on the displacement amount.
In accordance with vet another aspect of the invention, there is provided a mask alignment method for aligning a mask with respect to a projection optical system having an optical axis prior to transferring a pattern image of the mask onto a photosensitive substrate through the projection optical system. According to the method positions of projected images of at least two alignment marks formed on the mask are detected with the alignment marks having a predetermined positional relationship with the pattern image. A magnification of the projection optical system is changed, and the positions of the projected images are detected again. The mask position is adjusted based on the positions of the projected images determined before and after magnification.